The invention relates to apparatus and method for periodically sampling a liquid flowing in a pipeline.
The purpose of sampling a liquid is to obtain a representative portion of the liquid flowing through a pipeline from which the average composition of the flow can be determined. This is normally done by composing a set of individual sub samples taken with sufficient frequency to include all the effects of changing composition of the flow during the period of sampling. The more frequent the taking of these individual samples the more representative will be the composite sample.
The frequency of sampling may be at regular time intervals or at time intervals in inverse proportion to the rate of flow of the sampled liquid. The latter case being referred to as flow proportional sampling. When sampling crude liquids it is important to include any solids that are being carried by the liquid. In the particular case of crude oil the solids may be in the form of sand particulate matter or salt water or miscellaneous debris that have settled within the storage tanks. To obtain a representative sample of the solids or second phase liquids, the oil must be pumped at high velocity to prevent the settleable matter from being left behind and transported through pipe work of sufficient bore to avoid the risk of internal blockage within the apparatus.
Hitherto pipeline samples have incorporated a line filter to remove any solids material from the apparatus prior to the sampling. This was necessary because the bore size of such apparatus was insufficient to accomodate normal solids found in crude oil and hence were liable to blockage. This has been the prime limitation on existing products and which prevented them from accurately assessing the quality of crude oil. It has therefore been virtually impossible to access the true value of a cargo of crude oil being discharged from a super tanker. In many cases the normal cargo value would be of the order of $5,000,000 but may contain up to 2 percent by volume of contaminants in particular, the brine used as ballast from previous cargoes.